English I originates from Old English (OE) ic. Its predecessor ic had in turn originated from the continuation of Proto-Germanicik, and ek; ek was attested in the Elder Futhark inscriptions (in some cases notably showing the variant eka; see also ek erilaz). Linguists assume ik to have developed from the unstressed variant of ek. Variants of ic were used in various English dialects up until the 1600s.[1]

There is no known record of a definitive explanation from around the early period of this capitalisation practice.

It is likely[according to whom?] that the capitalization was prompted and spread as a result of one or more of the following:

changes specifically in the pronunciation of letters (introduction of long vowel sounds in Middle English, etc.)

other linguistic considerations (demarcation of a single-letter word, setting apart a pronoun which is significantly different from others in English, etc.)

problems with legibility of the minuscule "i"

sociolinguistic factors (establishment of English as the official language, solidification of English identity, etc.)

Other considerations include:

Capitalization was already employed with pronouns in other languages at that time. It was used to denote respect of the addresser or position of the addressed.

There is also the possibility that the first instances of capitalisation may have been happenstance. Either through chance or a sense of correctness, in the practice or the delivery, the capitalisation may have spread.

There are failings of many of these explanations based on other words, but there is the possibility that the factors or factor that prompted and/or spread this change may not have been applied to all similar words or instances.

According to traditional grammar, the objective case appears only as the direct object of a verb, the indirect object of a verb, or the object of a proposition. But there are examples which meet with varying degree of acceptance which violate this rule.

There are exceptions which appear with several pronouns:

it is me, as well as it is us/him/her/them.

Me and Bob are (the compound subject with a pronoun). This can be contrasted with the use of the subjective case as the object in to Bob and I

as me and than him (as if as and than were being treated as prepositions rather than as conjunctions)

These exceptions have their own exception: the objective case whom is never so used.

There are idiosyncratic uses generally restricted to the first person singular pronoun:

^ abThe genitives my, mine, thy, and thine are used as adjectives before a noun, or as possessive pronouns without a noun. All four forms are used as adjectives: mine and thine are used before nouns beginning in a vowel sound, or before nouns beginning in the letter h, which was usually silent (e.g. thine eyes and mine heart, which was pronounced as mine art) and my and thy before consonants (thy mother, my love). However, only mine and thine are used as possessive pronouns, as in it is thine and they were mine (not *they were my).

^ abFrom the early Early Modern English period up until the 17th century, his was the possessive of the third person neuter it as well as of the third person masculine he. Genitive "it" appears once in the 1611 King James Bible (Leviticus 25:5) as groweth of it owne accord.

Howe, Stephen (1996). The personal pronouns in the Germanic languages: a study of personal pronoun morphology and change in the Germanic languages from the first records to the present day. Studia linguistica Germanica 43. Walter de Gruyter. ISBN3-11-014636-3.